Multistage amplifiers are used in many applications where power is limited. For example, most space vehicles have only solar panels and batteries for gathering and storing power for all systems on the space vehicle. Space vehicles frequently have a communication system for communicating with earth or other space vehicles. The communication system has one or more power amplifiers for amplifying signals to be transmitted. The power amplifiers are ideally very small and very power efficient to conserve space and power on the space vehicle. The power amplifiers must also be able to amplify high frequency signals, often in the microwave and millimeter wave frequency bands. Monolithic microwave integrated circuit (MMIC) technology is often used to fabricate these power amplifiers in a small package. The power amplifiers may be fabricated using a gallium arsenide substrate to help meet stringent power, power efficiency and noise design requirements.
There are many techniques for improving the efficiency of power amplifiers. One technique for increasing power efficiency is conduction angle biasing. Conduction angle biasing generally involves amplifying only a portion of the input signal. Class AB amplifiers conduct and amplify between fifty and one hundred percent of the input signal. Class B amplifiers conduct and amplify about fifty percent of the input signal. Class C amplifiers conduct and amplify less than fifty percent of the input signal. A common characteristic of these conduction angle bias amplifiers is significant distortion of the output power relative to the input power. The distorted output provides the amplifier with undesirable amplitude modulation to amplitude modulation (AM-AM) and amplitude modulation to phase modulation (AM-PM) conduction and amplification characteristics.
Conventional power amplifiers such as Doherty amplifiers, Chireix out-phasing amplifiers and Envelope Elimination and Restoration (EER) amplifiers also suffer from poor AM-AM and AM-PM transfer characteristics. Moreover, these amplifiers and other conventional power efficient amplifiers are large, complex, or expensive and have limited input ranges and bandwidths.
Thus, there is a need for a power efficient amplifier having good AM-AM and AM-PM power transfer characteristics, but still keeping the amplifier small. The invention addresses this need as well as others.